Control and Monitoring Differentiated Service Code Point in Two-Way Active Measurement Protocol (TWAMP)ZTE Corp.gregimirsky@gmail.comEricssonsteve.baillargeon@ericsson.com
Transport
Network Working GroupInternet-DraftIPPMTWAMP DSCP
This document describes an optional extension for Two-Way Active Measurement Protocol (TWAMP)
allowing control and monitoring of the Differentiated Service Code Point (DSCP) field in forward and reverse directions
within single test session with the TWAMP-Test protocol.
This document, if accepted,
will be an update to the TWAMP core protocol specified in RFC 5357 and
DSCP Monitoring mode defined in RFC 7750
.
The One-Way Active Measurement Protocol (OWAMP) defines the Type-P Descriptor field and negotiation of its value in OWAMP-Control protocol.
The Two-Way Active Measurement Protocol (TWAMP) states that only a Differentiated Services Code Point (DSCP)
, , value can
be defined by Type-P Descriptor and the negotiated value must be used by both Session-Sender and Session-Reflector. The TWAMP specification also states
that the same DSCP value (found in the Session-Sender packet) MUST be used in the test packet reflected by the Session-Reflector.
The introduced optional DSCP Monitoring mode that can be negotiated using TWAMP Control
protocol and supported by TWAMP-Test protocol or by TWAMP Light mode. Still
the TWAMP-Test protocol does not support discovery of how Differentiated Services policies configured
along the IP path process various DSCP values in single test session.
Hence method defined in
can be characterized as per-session DSCP Monitoring. To provide higher efficiency and flexibility to monitoring
how Differentiated Services policies being applied this document proposes ability to control DSCP value to be used
by Session-Reflector for each TWAMP-Test packet. Such method can be characterized as per-packet DSCP monitoring
with TWAMP.
This document describes an OPTIONAL feature for TWAMP. It is called the DSCP and ECN Testing. It allows the
Session-Sender to use set of DSCP values through single test session and to instruct the Session-Reflector
on what DSCP value it must use for the reflected test packet. Furthermore this feature tracks the
Explicit Congestion Notification (ECN) , ,
value received at the Session-Reflector. This is helpful to determine if ECN is actually operating
or if an ECN-capable node has detected congestion in the forward direction.
DSCP: Differentiated Services Code Point
ECN: Explicit Congestion Notification
IPPM: IP Performance Metrics
TWAMP: Two-Way Active Measurement Protocol
OWAMP: One-Way Active Measurement Protocol
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
.
TWAMP connection establishment follows the procedure defined in
Section 3.1 of and Section 3.1 of where the Modes field
is used to identify and select specific communication capabilities. At the same time
the Modes field been recognized and used as an extension mechanism .
The new feature requires a new flag to identify the ability of a Session-Reflector
to support the new Session-Sender packet format in the TWAMP-Test protocol and
to use received DSCP and ECN values in the reflected to a Session-Sender test packet,
See the
for details on the assigned bit position.
The Server sets the DSCP and ECN Testing flag in the Modes field of the Server Greeting message
to indicate its capabilities and willingness to monitor them.
If the Control-Client agrees to test DSCP and ECN on some or all test sessions invoked with
this control connection, it MUST set the DSCP and ECN Testing flag in the Modes field in the Setup
Response
message.
Testing of DSCP and ECN requires support by the Session-Sender
and changes the test packet format in all the original
(unauthenticated, authenticated and encrypted) modes.
Testing of DSCP and ECN does not alter the Session-Reflector test packet format but certain considerations
must be taken when and if this mode is accepted in combination with Symmetrical Size mode
and/or with DSCP and ECN Monitoring mode .
When the Session-Sender supports DSCP and ECN Testing it constructs the Reflector
DSCP and ECN (R-DSCP-ECN) field, presented in
, for each
test packet it sends to Session-Reflector according to the following procedure:
value of the Reflector DSCP (R-DSCP) field MUST be set to the value that the Session-Reflector MUST use for the reflected test packet;
value of the Reflector ECN (R-ECN) field MUST be set to the value that the Session-Reflector MAY use for the reflected test packet.
When the Session-Reflector supports DSCP and ECN Testing mode it uses R-DSCP-ECN field of the received test packet to construct
the reflected test packet according to the following procedure
the R-DSCP field MUST be used as six (least-significant) bits of the Differentiated Service field of the reflected test packet;
the R-ECN field MAY be used as the two bits of the ECN field of the reflected test packet.
Formats of the test packet transmitted by the Session-Sender in unauthenticated, authenticated and encrypted modes
been defined in Section 4.1.2 . For the Session-Sender that supports
DSCP and ECN Testing these formats are displayed in
and .
For unauthenticated mode:
Neither Session-Reflector, nor Session-Sender in the DSCP and ECN Testing mode analyze, nor act on ECN value of the
received TWAMP test packet and therefore ignore congestion indications from the network.
It is expected that sending rates are low enough, as TWAMP deployment experience had demonstrated
since TWAMP base RFC 5357 publication in 2008, that ignoring these congestion indications
will not significantly contribute to network congestion.
For authenticated and encrypted modes:
defined DSCP and ECN Monitoring extension.
Using testing and monitoring modes in the same test session allows test DSCP in forward and reverse
directions because Session-Reflector returns received DSCP and ECN values in S-DSCP-ECN field in
the reflected test packet.
defined two extensions to TWAMP. First, to ensure that Session-Sender and Session-Reflector exchange
TWAMP-Test packets of equal size. Second, to specify number of octets to be reflected by Session-Reflector. If DSCP and ECN
Testing and Symmetrical Size and/or Reflects Octets modes are being negotiated between
Server and Control-Client in Unauthenticated mode, then, because R-DSCP-ECN field increases size of
unauthenticated Session-Sender packet by 4 octets, the Padding Length value SHOULD be >= 26 octets to allow for
the truncation process that TWAMP recommends in Section 4.2.1 of .
If DSCP and ECN Testing mode to be used in combination with Symmetrical Size
and DSCP and ECN Monitoring modes, then the Padding Length value SHOULD be >= 27 octets
to allow the truncation process that TWAMP recommends in Section 4.2.1 of .
Appendix I of does not explicitly state how the value of the Type-P Descriptor
is
synchronized between Session-Sender and Session-Reflector and whether different values are considered
as error condition and should be reported.
In order to test DSCP over round-trip path between Session-Sender and Session-Reflector it is sufficient that
Session-Reflector uses received DSCP value for the reflected test packet. If the Session-Reflector supports
both testing and monitoring of DSCP, then TWAMP Light mode MAY be used to test DSCP in forward and reverse directions.
The TWAMP-Modes registry defined in .
IANA is requested to reserve a new DSCP and ECN Testing Capability as follows:
Bit DescriptionSemantics Definition ReferenceTBADSCP and ECN Testing Capability This document
Testing of DSCP and ECN does not appear to introduce any additional security threat to hosts that
communicate with TWAMP as defined in , and existing extensions .
Sections such as 3.2, 4., 4.1.2, 4.2, and 4.2.1 of discuss
unauthenticated, authenticated, and encrypted modes in varying degrees of detail.
The security considerations that apply to any active measurement of live networks are
relevant here as well. See the Security Considerations sections in , , and
.
TBD